Method of depositing films of material



May 20, 1947. m; 2,420,724

METHOD OF DEPOSITING FIL IS 0F MATERIAL Filed Sept. 21, 1944 l2 26 w E 3 l3 IO -2| 28 as I zo as r 33 28 RICHARD M. RICE (Ittomeg Patented May 20, 1947 UNITED STATES PATENT OFFICE Richard M. Rice, Rochester, N. -Y., assignor to Bausch &'Lomb Optical Company, Rochester, N. Y., a corporation of New York Application September 21, 1944, Serial No. 555,097

thin films of material on a surface of a bodyin an evacuated chamber and more particularly it has reference to methods for treating the surface before. during, and after the deposition of the film thereon.

I One of the objects of this invention is to provide a method'whereby films deposited in an evacuated chamber will be more durable and resistant to corrosion and mechanical abrasion. A further object is to provide a method for treating a surface of a, body by electronic bombardment prior to the deposition of the film thereon. Another object is to provide a method whereby thin films of material may be deposited on the surface of a body in an evacuated chamber without heating the entire body so that metallized or cemented optical elements may, for example, be coated with such films without causing deterioration of the cement or metal. Still another object is to provide a method for depositing a, thin film of material on a recessed surface of a body whereby the surfaces of optical elements may readily be coated uniformly in commercial production even though the elements are secured within mountings.

These and other objects and advantages reside in certain novel methods and processes and the steps of carrying out such methods and processes as will hereinafter be more fully described and pointed out in the appended claims. The figure of the drawing shows a schematic diagram of one type of apparatus which may be used in practising my invention.

While the invention maybe carried out with various types of apparatus, there is shown in the figure a, preferred form of apparatus for the practise of the invention. A metal base plate I supports a bell jar II which is sealed thereto by means of any suitable sealing material l2 so as to provide an air-tight joint. Secured to base plate ill by any suitable means are the spaced members l3 which support insulating members It carrying metal supports i5. Mounted on the supports I5 is a metal plate l6 having spaced apertures 11 in which are mounted the objects to be coated, such as lenses I8, which of the conductor 23.

8 Claims. (Cl. 117-54) Mounted within-the bell jar II is the heating filament 24, formed of tungsten wire, for vaporizing the material which is to be deposited on the surfaces of lenses 18. The material to be deposited may be supported on a molybdenum strip, inserted within the coils of filament 24 or by any other means well-known to those skilled in the art. The ends of the filament 24 are connected to a suitable source of current by conductors 25 passing through insulating bushings 26 in the base plate ID. A second filament 21 is also mounted within the bell jar II and one of its ends is connected to a source of current by conductor 28 passing through insulating bushing 29 in the base plate It while the other end of the filament is grounded to the base plate ill, the other side of the current source being grounded by conductor 28' to plate "I. The chamber formed by the bell ar H and base plate III may be evacuated by any suitable pumping means, indicated at 33, connected to the chamber by the pipe 33'.

In one preferred method of practising the invention the lenses I8 are secured to the plate It so that the surfaces to be coated face the filament 24. The chamber is then evacuated to a pressure of approximately 50 microns and the transformer 20 is energized to supply a potential of about 1,200 volts between the plate l0 and the holder l6. This discharge serves to degas the chamber and the parts therein and it is a process which is commonly used in the art.

Following the degassing operation, the chamber is evacuated to a pressure of about 0.1 micron of mercury and the potential between plate I0 and lens holder I6 is increased to about 3,000 volts. The filament 21 is then energized so that it emits electrons which are attracted to the plate l6 during the positive half of the current cycle. At pressures of 10 microns or less, little, if any, current would fiow from plate ID to the lens holder l6. However, by providing a source of electrons, such as the hot filament 21, within the chamber, it is possible for current to fiow from the plate I0, acting as one electrode, to the plate It, which acts as the other electrode, even if the pressure in the chamber be as low as 10 microns or less. After the filament 2'! has been energized for a period of about eight to ten minutes, the filament 24 is energized so as to evaporate the material carried thereby. When the material has been evaporated and condensed to a desired thickness on the lenses l8, the current is cut off from the filament 24 but the emission of electrons from filament 21 is 3 continued for two or three minutes longer. All current supplies are then cut oil, the vacuum released, and the coated lenses removed.

The method just described has been successfully used to produce thin films of magnesium fluoride, calcium fluoride, and zinc sulfide on the surfaces of lens elements, Such films are highly resistant to chemical action and corrosion, such as produced by a salt water spray, and they will also withstand abrasion produced by rubbing wet steel wool over the coated surfaces. Various types of either metallic or non-metallic materials may be evaporated and the beneficial results of the new method will be obtained in each case.

While, in the example given above, it is stated that the chamber is evacuated to a pressure of about 0.1 micron before the electrical discharge treatment, such a pressure is not critical but may range anywhere from about 0.01 micron to microns. Similarly, the voltage which is supplied by the high voltage side of the transformer may range from about 1,000 to 15,000 volts. The length of time during which the electronic bombardment or electrical discharge treatment is carried out, before depositionof the film, may also be varied from about five to ten minutes. Although in the preferred method the electronic bombardment is carried on before, during, and after the deposition of the film, it is not essential that it be carried on during or after evaporation. It is essential. however, that the electronic bombardment or electrical discharge treatment of the surface to be coated should be carried out immediately prior to the deposition of the film.

As stated above, practise in the art of degassing the evacuated chamber and article before the coating operation takes place is well known. But such preliminary treatments do not produce the results which can be attained by this invention. Under one such prior art practise, the chamber and article are degassed, prior to the deposition of the film, by an electrical discharge produced by a relatively low voltage of the order of 2,000 volts while the chamber has a relatively high pressure of the order -50 microns. Under another prior art method the chamber and the article to be coated are degassed or treated with heat which may be provided by infra red sources within the chamber or by other suitable means.

Such heating steps are usually carried out both before and during the deposition of the thin film. When this latter method is used, the article to be coated, such as a lens element, for example, becomes heated throughout. Accordingly, with such a process it is impossible to deposit thin films on lens elements which have been cemented or metallized because the heating will cause a deterioration of the cement or metallized layers.

It is frequently necessary to apply a thin film of material to the outer surfaces of optical elements which are in optical contact with each other. This cannot be done under the prior art practises because the heat applied would cause the elements to spring apart. With the method of this invention, however, the surfaces of the article to be coated are only locally heated and hence the article, such as a glass lens, for example, does not become heated through. Consequently, with my invention it is possible to deposit durable thin films on optical elements which have been cemented, metallized, or placed in optical contact with each other.

Sometimes it is necessary to deposit thin films of material on a recessed surface of a body, such as a surface of a lens which is secured in a mounting. Under the prior art production practises, it is difficult to deposit uniform films of material on such mounted lenses because the mounting tends to obstruct the passage of some of the particles of material to certain parts of the surface of the lens during the evaporation or sputtering processes. The deposition of thin films of material on mounted lenses, in commercial production, is, however, greatly facilitated by the use of my electronic bombardment or electrical discharge treatment. When coating such mounted lenses under my method, it is necessary that the electronic bombardment be continued during the evaporation.

From the foregoing it will be apparent that the objects of the invention are attained and that there are provided improved methods for depositing thin films of material on the surface of a body. The films deposited by the use of this method are resistant to corrosion such as is produced by salt water, for example. The films are also durable since they will withstand abrasion produced by rubbing the coated surface with wet steel wool. Although a transformer 20 has been shown for supplying a potential between the plate It and the plate II], it will be obvious that a suitable source of direct current of the proper voltage could be used in place of the transformer. It is also obvious that the method could be carried out by other types of apparatus than that shown in the figure of the drawing. Various other modifications could be made without departing from the spirit of the invention as pointed out in the appended claims.

I claim: I

l. A method of depositing a film of materialon the surface of a body in an evacuated chamber whichv comprises treating the surface before deposition of the film thereon by applying a potential to electrodes within the chamber and providing a thermal source of electrons in the chamber while holding the pressure within the chamber, at about 0.01-10 microns and thereafter depositing the film on the surface in the evacuated chamber.

2. A method of depositing a thin film of material on a surface of a body in an evacuated chamber which comprises the steps of bombarding the surface with a stream of electrons while maintaining the pressure in the chamber at about 0.01-10 microns and immediately thereafter and under substantially the same pressure deposiitng the material on the surface.

3. The method of depositing a film of material on a surface of a body in an evacuated chamber which comprises evacuating the chamber to a pressure of about 0.01-10 microns, creating and maintaining an electrical discharge within the chamber by applying a potential to electrodes in the chamber and providing a thermal source of electrons in the chamber and immediately after the discharge treatment depositing a film of the material on the surface within the chamber.

4. A method of depositing a thin film of material on a surface of a body in an evacuated chamber by thermal evaporation which comprises evacuating the chamber to a pressure of about 0.01-10 microns, subjecting the surface to an electrical discharge by applying a potential to electrodes in the chamber and providing a thermal source of electrons in the chamber, and immediately after the discharge vaporizing the material so that it is condensed on the surface.

5. The method of depositing a film of material on a surface of a body in an evacuated chamber by thermal evaporation which comprises subjecting the surface to electronic bombardment for at least 5 minutes while maintaining the pressure in the chamber at about 0.01- microns,

evaporating the material and continuing the bombardment while the material is being evaporated and condensed on the surface.

6. The method of depositing a. film of material by thermal evaporation on a surface of a body held by a carrier in an evacuated chamber which.

comprises subjecting the surface to electronic bombardment for 5-10 minutes by electrically charging the carrier with a potential of 1,000- 15,000 volts and providing a thermal source of electrons in the chamber while maintaining the pressure in the chamber at about 0.01-10 microns, evaporating the material and continuing the bombardment while the material is being evaporated and condensed on the surface and for 1-3 minutes after a film of the desired thickness has been deposited on the surface.

7. The method of depositing a film of material by thermal evaporation on a surface of a body held byca carrier in an evacuated chamber which comprises subjecting the surface to electronic bombardment for 5-10 minutes by electrically charging the carrier with a potential of 1,000- 15,000 volts and providing a thermal source of electrons in the chamber while maintaining the pressure in the chamber at about 0.01-10 microns and immediately thereafter evaporating the material so that it is condensed on the surface.

8. A method of depositing a film of material by thermal evaporation on a surface of a. body held by a carrier in an evacuated chamber which comprises the steps of subjecting the surface to electronic bombardment for at least 5 minutes by electrically charging the carrier with a potential of at least 5,000 volts and providing a thermal source of electrons within the chamber ,while maintaining the chamber at a pressure of about 0.1 micron, evaporating the material within the chamber until a film of the desired thickness is deposited on the surface and continuing the horn. bardment during the evaporation process.

RICHARD M. RICE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,965,587 Foulke July 10, 1934 2,067,907 Edwards Jan. 19, 1937 2,164,595 Siebertz July 4, 1939 2,103,623 Kott Dec. 28, 1937 2,157,478 Burkhardt et a1. May 9, 1939 2,322,613 Alexander June 22, 1943 FOREIGN PATENTS Number Country Date 510,404 Great Britain Aug. 1, 1939 

